Automatic gain method and controller for mass spectrometer

ABSTRACT

A method and apparatus for analyzing relative abundance of the composition of a sample with a mass spectrometer having an electron multiplier controlled with a high voltage supply. When the sample is a gas mixture, the volume per cent of a gas is analyzed. The all-ion peak signal is repetitively sampled and held to produce an analog output voltage. This output voltage is compared with a reference voltage via an error amplifier in a feedback loop to control the high voltage applied to the electron multiplier so that the all-ion peak signal of the mass spectrometer is maintained a constant.

United States Patent [191 [in 3,823,315 Mosharrafa July 9, 1974AUTOMATIC GAIN METHOD AND 3,648,047 3/1972 Bushman 250/4l.9 D

CONTROLLER FOR MASS SPECTROMETER [75] Inventor: Mostafa A. Mosharrafa,Edina,

Minn.

[73] Assignee: Automated Medical Systems, Inc.,

Minneapolis, Minn. [22 Filed: Oct. 30, 1972 [2l] Appl. No.: 302,256

[52] US. Cl. 250/283, 250/292 [5 1] Int. Cl. H0lj 39/34, BOld 59/44 [58]Field of Search. 250/4l.9 D, 41.95 R

[56] References Cited UNITED STATES PATENTS 3,510,647 5/!970 Wood250/4l.9 D 3,641,340 2/1972 Van Der Grinten 250/4l.9 D

Primary Examiner-James W. Lawrence Assistant ExaminerC. E. Church [5 7]ABSTRACT A method and apparatus for analyzing relative abundance of thecomposition of a sample with a mass spectrometer having an electronmultiplier controlled with a high voltage supply. When the sample is agas mixture, the volume per cent of a gas is analyzed. The all-ion peaksignal is repetitively sampled and held to produce an analog outputvoltage. This output voltage is compared with a reference voltage via anerror amplifier in a feedback loop to control the high voltage appliedto the electron multiplier so that the all-ion peak signal of the massspectrometer is maintained a constant.

13 Claims, 1 Drawing Figure GAS CHANNELS ELECTRON ION SOURCE QUADRUPOLEFILTER MULTIPLIER I V ALL-|ON PiEAK ALL'ION CHANNEL H.V. l5 CHANNEL GASSELECTOR SUPPLY M 74 VOLTAGE 23 REGULATOR SAMPLE HOLD REF 0 /9 ZPATENTED JUL 91974 I AUTOMATIC GAIN METHOD AND CONTROLLER FOR MASSSPECTROMETER BACKGROUND OF THE INVENTION A common non-magnetic massspectrometer employs a quadrupole radio frequency filter for massanalysis of ions. This instrument is capable of scanning the massspectrum in a few milliseconds and can provide a convenient visual realtime observation of the mass spectrum on an oscilloscope. The instrumentis sensitive to ion source pressure fluctuations, ion beam densityfluctuations, and electronic gas signal drifts and fluctuations. Theinstrument does not measure directly relative abundance of thecomposition of a sample, as gas, and accordingly is not a volume percent analyzer.

SUMMARY or THE INVENTION The invention is directed to a method andapparatus for analyzing relative abundance of the composition of asample, as ages, with a mass spectrometer system using an electronmultiplier and having a signal proportional to the total ion densityavailable for control. The all-ion peak of a sample is held and averagedwith a time constant to produce an output signal. The output signal ofthe sample is compared with a constant reference voltage applied to anerror amplifier. The output of the error amplifier is used to controlthe high voltage applied to the electron multiplier whereby the all-ionpeak of the spectrometer is maintained a constant. This techniquetransforms the mass spectrometer from a partial pressure analyzer to avolume per cent analyzer. In other words, the mass spectrometer nolonger measures partial pressure, but relative abundance of acomposition of a sample, as a gas. In addition, the technique greatlyimproves the overall stability of the mass spectrometer.

IN THE DRAWINGS FIG. 1 is a schematic block diagram of a massspectrometer and the means and method of the invention whereby thespectrometer operates to analyze the volume per cent of a gas.

Referring to FIG. I, there is shown a mass spectrometer indicatedgenerally at usable to provide output information concerning therelative abundance of a gas in a mixture of gases. Spectrometer 10 is anonmagnetic instrument that has an ion source 11 and employs aquadrupole radio frequency filter 12. Filter 12 is coupled to anelectron multiplier 13. A multichannel gas selector 14 controls theoperation of the'filter 12 for repetitive sampling of the selectedgases. An additional channel 16 is added to sample repetitively theall-ion peak output signal. The all-ion output peak signal is the totalion current after amplification of the electron multiplier when nofiltering is made by the analyzer or filter. Gas selector 14 has one ormore channels connected to the filter. For example, selector 14 has fivechannels.

The electron multiplier 13 is controlled with a high voltage supply 17.The power input to the high voltage supply 17 is controlled with avoltage regulator circuit 23. A plurality of gas channels 18 areconnected to the electron multiplier 13.

A ion peak output signal as the all-ion peak output I signal, of theelectron multiplier 13 is used in conjunction with a feedback loopindicated generally at 19 to control the gain of the electron multiplier13 thereby transforming the mass spectrometer from a partial pressureanalyzer to a volume per cent analyzer when the sample is a gas. Themass spectrometer 10 with feedback loop 19 analyzes relative abundanceof the composition of a sample. Feedback loop 19 includes the all-ionsignal sample-and-hold circuit 20, with or without filtering. The outputof the sample-and-hold circuit 20 is a voltage signal proportional tothe all-ion peak intensity. Circuit 20 is a means to measure the allionpeak of a sample. This means can be a pressure gauge, ionization gauge,the output of a chromatograph, theaIl-ion peak of a quadrupole gaschromato graph or like instruments to measure the all-ion peak. Theoutput signal of circuit 20 is applied to an error amplifier 21 togetherwith a reference voltage 22. The

sample signal is compared with the constant reference voltage 22 by theerror amplifier 21. The output signal of the error amplifier 22 drivesthe voltage regulator circuit 23, thereby controlling the power input tothe high voltage power supply 17, which in turn controls the highvoltage applied to the ion multiplier 13. The result is that the masspeak heights of the spectrometer 10 become independent of the pressurein the ion source and the spectrometer no longer measures partialpressures but relative abundances of the components of a sample, as agas, introduced into the ion source of the mass spectrometer.

In terms of a method of analyzing relative abundance of the compositionof a sample, as the volume per cent of a gas, the feedback loop 19 isused in conjunction with the mass spectrometer or instrument 10 havingan electron multiplier 13 and a high voltage supply 17 controlling theelectron multiplier. The method comprises repetitively sampling theall-ion or selected ion peak outputsignal of the electron multiplier 13of the gas. A sample of the all-ion peak output signals is held andaveraged with a time constant producing a sample output signal. Theoutput signal proportional to the allion peak is compared with aconstant reference voltage with the use of an error amplifier 21. Theoutput of the amplifier is used to control the high voltage applied tothe electron multiplier whereby the all-ion peak output signals of thespectrometer are maintained constant. The output of the amplifier cancontrol voltage regulator circuit 23 which controls the power input tohigh voltage supply 17. This transforms the mass spectrometer from apartial pressure analyzer to a volume per cent analyzer. The method canbe applied to any mass spectrometer system using an electron multiplierand a signal proportional to the total ion density. The total iondensity signals may be derived from an ionization gauge or a gaschromatograph detector.

The measurement of the relative abundance of the gas in accordance withthe structure and method of the invention eliminates the effects of ionsource pressure fluctuations due to outgassing, variations in samplesize, and differences in pumping speeds. Also, there is an eliminationof pressure regime changes in continuous sampling inlet systems forgases due to changes in viscosity of the gas sample, changes in pressuregradient due to condensable vapors and changes in flow regime, i.e.,transonic flow and capillary tubing and the like. In addition, theeffects of ion beam density fluctuations are eliminated. Thesefluctuations are due to emission current changes, ion source changes,ion

source sensitivity degradation due to contamination and ion sourcethermal nonequilibrium. Furthermore, electronic gas signal drifts andfluctuations are eliminated. These fluctuations are due to changes inelectron multiplier gain and drifts in the electrometer amplifier.

While there have been shown and described a preferred method andapparatus for analyzing relative abundance of the composition of asample, as the volume per cent of a gas, it is understood that changesand modifications in the method and apparatus can be made by thoseskilled in the art without departing from the spirit of the invention.The invention is defined in the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of controlling the sensitivity of a mass spectrometer havingan electron multiplier producing ion output signals and a high voltagesupply controlling the electron multiplier comprising: measuring aselected ion peak with the electron multiplier to provide an outputsignal, averaging a sample of said output signal with a time constantrepresentative of the period during which said selected ion peak ismeasured to provide a sample output signal, comparing the sample outputsignal with a constant reference voltage by applying said output signaland said reference voltage, to an error amplifier, and controlling thehigh voltage applied to the electron multiplier with the output of theerror amplifier so that the selectedion peak sample output signals ofthe spectrometer are maintained constant. Y

2. The method of claim 1 wherein: the output of the amplifier drives avoltage regulator circuit that controls the power input to the highvoltage power supply.

3. The method of claim 1 wherein: the selected ion peak output signal isthe all-ion peak output signal of the electron multiplier.

4. A means for controlling the sensitivity of a mass spectrometer havingan ion source, filter means, an electron multiplier receiving ions fromsaid source, and high voltage power supply means for controlling saidelectron multiplier, the improvement comprising means for holding andaveraging over a period of time a sample of a selected ion peak outputsignal of the electron multiplier so as to produce a sample outputsignal, and error amplifier means for receiving the sample output signaland a reference voltage, said amplifier means having an outputcontrolling the high voltage applied to the electron multiplier suchthat the sample ion output signals of the spectrometer are maintainedconstant.

5. The means of claim 4 including: circuit means for controlling thepower input to the high voltage power supply means thereby controllingthe high voltage applied to the electron multiplier.

6. A means for improving the stability of a mass spectrometer having anion source, electron multiplier producing ion output signals, and highvoltage source means supplying high voltage to the electron multipliercomprising: first means for sampling a selected ion output signal of theelectron multiplier to provide a sample output signal, and a secondmeans for comparing said sample output signal with a constant referencesignal and applying the difference between the sample output signal andreference signal to a closed feedback loop to control the gain of theelectron multiplier by controlling the high voltage source means to varythe high voltage applied to said electron multiplier.

7. The means of claim 6 wherein: said first means is an instrument tosample the all-ion peak of a substance.

8. The means of claim 6 wherein: said second means includes an erroramplifier.

9. The means of claim 6 wherein: said second means includes circuitmeans for controlling the power input of the mass spectrometer.

10. The means of claim 6 wherein: said second means includes an erroramplifier and circuit means connected to the error amplifier forcontrolling a high voltage supply for the mass spectrometer.

11. A mass spectrometer for analyzing relative abundance of thecomposition of a sample comprising: an ion source, filter means forreceiving ions from said ion source, electron multiplier means forreceiving ions from said filter means, a high voltage supply forcontrolling the electron multiplier means, means for holding andaveraging over a period of time a sample of a selected ion peak outputsignal of the electron multiplier means thereby producing a sampleoutput signal, and error amplifier means for receiving the sample outputsignal and a reference voltage, said amplifier means having an outputcontrolling the high voltage supply to vary the high voltage applied tothe electron multiplier means so that the sample output signals of thespectrometer are maintained constant.

12. The mass spectrometer of claim 11 including: circuit means forcontrolling the power input to the high voltage supply therebycontrolling the high voltage applied to the electron multiplier means.

13. The mass spectrometer of claim 11 wherein: the

filter means is a quadrupole filter.

1. A method of controlling the sensitivity of a mass spectrometer having an electron multiplier producing ion output signals and a high voltage supply controlling the electron multiplier comprising: measuring a selected ion peak with the electron multiplier to provide an output signal, averaging a sample of said output signal with a time constant representative of the period during which said selected ion peak is measured to provide a sample output signal, comparing the sample output signal with a constant reference voltage by applying said output signal and said reference voltage, to an error amplifier, and controlling the high voltage applied to the electron multiplier with the output of the error amplifier so that the selected ion peak sample output signals of the spectrometer are maintained constant.
 2. The method of claim 1 wherein: the output of the amplifier drives a voltage regulator circuit that controls the power input to the high voltage power supply.
 3. The method of claim 1 wherein: the selected ion peak output signal is the all-ion peak output signal of the electron multiplier.
 4. A means for controlling the sensitivity of a mass spectrometer having an ion source, filter means, an electron multiplier receiving ions from said source, and high voltage power supply means for controlling said electron multiplier, the improvement comprising means for holding and averaging over a period of time a sample of a selected ion peak output signal of the electron multiplier so as to produce a sample output signal, and error amplifier means for receiving the sample output signal and a reference voltage, said amplifier means having an output controlling the high voltage applied to the electron multiplier such that the sample ion output signals of the spectrometer are maintained constant.
 5. The means of claim 4 including: circuit means for controlling the power input to the high voltage power supply means thereby controlling the high voltage applied to the electron multiplier.
 6. A means for improving the stability of a mass spectrometer having an ion source, electron multiplier producing ion output signals, and high voltage source means supplying high voltage to the electron multiplier comprising: first means for sampling a selected ion output signal of the electron multiplier to provide a sample output signal, and a second means for comparing said sample output signal with a constant reference signal and applying the difference between the sample output signal and reference signal to a closed feedback loop to control the gain of the electron multiplier by controlling the high voltage source means to vary the high voltage applied to said electron multiplier.
 7. The means of claim 6 wherein: said first means is an instrument to sample the all-ion peak of a substance.
 8. The means of claim 6 wherein: said second means includes an error amplifier.
 9. The means of claim 6 wherein: said second means includes circuit means for controlling the power input of the mass spectrometer.
 10. The means of claim 6 wherein: said second means includes an error amplifier and circuit means connected to the error amplifier for controlling a high voltage supply for the mass spectrometer.
 11. A mass spectrometer for analyzing relative abundance of the composition of a sample comprising: an ion source, filter means for receiving ions from said ion source, electron multiplier means for receiving ions from said filter means, a high voltage suppLy for controlling the electron multiplier means, means for holding and averaging over a period of time a sample of a selected ion peak output signal of the electron multiplier means thereby producing a sample output signal, and error amplifier means for receiving the sample output signal and a reference voltage, said amplifier means having an output controlling the high voltage supply to vary the high voltage applied to the electron multiplier means so that the sample output signals of the spectrometer are maintained constant.
 12. The mass spectrometer of claim 11 including: circuit means for controlling the power input to the high voltage supply thereby controlling the high voltage applied to the electron multiplier means.
 13. The mass spectrometer of claim 11 wherein: the filter means is a quadrupole filter. 